1. Field of the Invention
The present invention relates to a projection display system, and more particularly to a projection display system with a reflective light valve.
2. Description of Related Art
A conventional projection display system is to use a light beam emitted from a light source to converge on one or more light valves to allow the beam to carry signals of images, and be then projected onto a screen to form images through a projection lens. However, because a multimedia presentation prevails, for attaining to a convenient carrying, the projection display system and device tend to be light, thin, short and small, a single reflective light valve is adopted in a general display device for presentation use.
A projection display system 10 with a single reflective light valve disclosed in U.S. Pat. No. 5,552,922, as FIG. 1 shows, has a light source 11, a prism 12, a light valve 13 and a projection lens 14, in which the prism 12 has two planes 121 and 122 parallel to each other. The light valve 13 and the projection lens 14 are disposed adjacent to the planes 121 and 122 of the prism 12. The light valve 13 is a digital micro-mirror device (DMD). The light source 11 provides an illumination beam incident into the prism 12, and the beam is then projected on the light valve 13 by means of total reflection. Thereafter, the beam reflected from the light valve 13 is passed through the prism 12 and enters the projection lens 14 to project onto a screen (not shown) to form images. Because the prism 12 is installed between the light valve 13 (i.e. an image formation surface) and the projection lens 14 (i.e. a lens system) in the above-mentioned system, a back focal length (BFL) defined a distance from the last surface of the lens system to the image formation surface thereof is caused to be large. And, if the back focal length is lager, an aberration of the images formed in the projection lens 14 will be caused to be large, and a larger size of the projection lens 14 will be needed such that the size of the whole system cannot be reduced.
Another projection display system 10 with a single reflective light valve disclosed in U.S. Pat. No. 5,309,188, as FIG. 2 shows, has a coupling prism assembly 22 installed between a light valve 23 and a projection lens 24, the coupling prism assembly 22 consists of an isosceles right triangle prism 221 and a wedge prism 222. One side of the wedge prism 222 is adjacent to the hypotenuse of the isosceles right triangle prism 221. The light valve 23 is installed adjacent to one leg of the isosceles right triangle prism 221. A light beam provided from a light source 21 is incident on the light valve 23 after passed through the coupling prism assembly 22. The light beam reflected from the light valve 23 is reflected via the hypotenuse of the isosceles right triangle prism 221 and then projected into the lens 24, and images are finally displayed on a screen (not shown). Although the light valve 23 and the lens 24 are perpendicularly disposed in the system mentioned above, the coupling prism assembly 22 still causes that a back focal length cannot be shortened effectively.
It is concluded that the problems, that the back focal length is still too long and the volume of the projection lens is too large, still exists in the projection display system with a current single plate reflective light valve. According, the present invention is proposed to solve such kinds of problems.